This has just popped into my head, I need to mull it a bit more before I'm convinced I believe it:
the how-not-to-do-it examples in your and Sanderson's descriptions have two salient qualities: arbitrariness and effortlessness. The former, I think, is a matter of tone more than anything else. Magic needs to feel consistent, it does not need to necessarily be consistent.
The deus-ex-machina climaxes that Sanderson warns against are really more about that effortlessness. An example: in WoT, which, as I said, is generally too Hard for my tastes, there's a wicked important climactic bit where the main protaganist attempts a massive alteration of the very nature of magic itself. It is a very Soft moment--there had been no prior indication that human effort could produce any such effect.
However, Jordan emphasizes what a huge amount of work, skill, suffering, and risk it requires, and that's what makes it not feel like a cheat.
Conclusion: magic has to cost somehow. I suspect that this is why so much of the magic in Harry Potter felt flat to me--Rowling made so much of it costless & effortless.
My New Theory
the how-not-to-do-it examples in your and Sanderson's descriptions have two salient qualities: arbitrariness and effortlessness. The former, I think, is a matter of tone more than anything else. Magic needs to feel consistent, it does not need to necessarily be consistent.
The deus-ex-machina climaxes that Sanderson warns against are really more about that effortlessness. An example: in WoT, which, as I said, is generally too Hard for my tastes, there's a wicked important climactic bit where the main protaganist attempts a massive alteration of the very nature of magic itself. It is a very Soft moment--there had been no prior indication that human effort could produce any such effect.
However, Jordan emphasizes what a huge amount of work, skill, suffering, and risk it requires, and that's what makes it not feel like a cheat.
Conclusion: magic has to cost somehow. I suspect that this is why so much of the magic in Harry Potter felt flat to me--Rowling made so much of it costless & effortless.